This invention relates to electronic watthour meters, and more particularly to an electronic watthour meter in which its light load characteristic is improved and its characteristic with time is stabilized.
Since an electronic watthour meter has no mechanically movable components, its characteristic with time is more excellent than an induction type watthour meter which is essentially formed with mechanical components. Furthermore, its circuit is composed by integrated circuits, so that its size is small, it is suitable for mass production and the manufacturing cost can be reduced.
Accordingly, it seems that an electronic watthour meter will eventually replace the induction type watthour meter and will be the main type of watthour meter. A variety of electronic watthour meters have been proposed in the art.
These conventional electronic watthour meters have an equivalent circuit as shown in FIG. 1. Thus, the watthour meter comprises: a multiplication circuit 10 in which a voltage signal e.sub.v, proportional to the load voltage of the power supply lines, and a voltage signal e.sub.i, proportional to the consumption current of the power supply lines, are subjected to multiplication thereby to provide a voltage signal .sup.e o (.sup.e o=k.multidot.e.sub.v .multidot.e.sub.i, where k is a constant) proportional to the instantaneous power of the supply lines; and a voltage-to-frequency converter 11 in which the output voltage signal .sup.e o, of the multiplication circuit 10, is subjected to integration to provide a frequency signal f.sub.out. Accordingly, the integrated power value of the supply lines can be obtained by counting the frequency signal f.sub.out outputted by the voltage-to-frequency converter 11.
The accuracy of the watthour meter is expressed by the absolute error with respect to a true measurement value instead of relative error with respect to the full scale (rating). In the watthour meter, the multiplication circuit 10 and the voltage-to-frequency converter 11 are made up of operational amplifiers. To obtain the high accuracy required by a watthour meter, an offset voltage of the multiplication circuit and the voltage-to-frequency converter must be suppressed within an extremely small value of voltage when the input of the watthour meter is small and its load is light. It is necessary that the offset voltage should be suppressed within the absolute error even if the input of the watthour meter is 1/30 (3.33%) of the rating (100%), 1/50 (2%) or 1/100 (1%). If it is assumed that the rating of the voltage signal Vi, proportional to the consumption current, is 5 V for instance, then the error of 0.5% corresponds to an input 25 mV in conversion value, with the rating being 100%. Therefore, if the input in conversion value is on the order of 25 mV, the accuracy will not be so seriously affected. However, the input in conversion value is 0.5 mV in the case of 0.5% error with respect to a 1/50 input. Accordingly, it is necessary that the offset voltage induced by the operational amplifiers in the multiplication circuit 10 and the voltage-to-frequency converter 11 will be reduced less than 0.5 mV. However, it is quite difficult to suppress the offset voltage within an extremely small value range or to eliminate it. In addition, the offset voltage is varied with time and temperature, so that the accuracy of measurement of power tends to be low.